Contemporaneous with the commercialization of electrostatic copiers, there has been a desire to increase the capability of the copier machine without, at the same time, degrading its performance. One particularly desirable feature which has been introduced is the capability of reducing the image size in relation to the size of the original document. The advent of copiers capable of this reducing function required the solution of several problems, i.e., those particularly caused by changes induced as a result of the changes in the optical configuration required to reduce the image. While the solution of these problems in a laboratory environment may be trivial, the constraints imposed by the commercialization of these devices made the solution of these problems more difficult. In particular, the commercial device capable of reduction must exhibit the same image sharpness and consistency of image exposure as a nonreduction machine with desirably little or no increase in equipment size, cost or maintenance difficulty.
While a copier capable of reducing an image to a particular ratio satisfies more of the user's needs than a machine which is not so capable, it is also desirable to increase the number of reduction modes and finally to provide for continuously variable reduction within some specified range of reduction modes. In connection with this description, a reduction mode is defined as a machine configuration to produce a specified reduction ratio, not equal to 1. As the number of reduction modes is increased until it becomes essentially continuous, the number of optical problems to be solved increase, and with the constraints imposed on commercial devices, the difficulty in solving these problems increases.
Desirably, the image produced by a copier is uniform in exposure, and the achievement of this uniformity requires careful design. For example, the presence of a lens in the optical path results in image irradiance reduction for that portion of the image passing through the lens off the optical center line, i.e., so-called cos.sup.4 losses. In the piror art, solutions to this difficulty have been achieved by shaping the object irradiance so as to compensate for these lens effects and similar shaping has been used to compensate for otherwise uneven object irradiance.
However, the introduction of a reduction capability caused further variations in the image exposure since, as reduction is introduced, image irradiance at the image plane increases. The variations in exposure in a machine which included a single reduction mode (i.e., a reduction ratio other than 1) had been compensated for in the prior art by adding an aperture only in the reduction mode to limit image exposure in that mode. This aperture, mask or light stop, could theoretically be located either adjacent the image plane or adjacent the object plane, and in the case of its location near the object plane, it could be located between the source of irradiance and the object or between the object and the lens.
Compounding the problem is the fact that an elongated light source produces more light toward the center of the source than at the edges and the additional fact that light rays are received with more irradiance at the center of a curved drum surface than at the edges.
A further complication arises in some machines which are capable of reduction by reason of the relationship between the center line of objects of different size. In one group of machines, the center line is not changed, i.e., the objects are center-referenced; obviously, this causes no additional difficulties. However, in another group of machines, the objects to be copied are corner-referenced, and as a result, as the object to be copied increases in size, and the reduction mode is correspondingly changed, the center line moves or changes in position relative to the center line of a smaller object to be copied. This "corner-referencing" serves to increase the difficulties associated with cos.sup.4 losses and other irradiance distortions, since more of the image to be reproduced falls in the edge where image exposure is reduced without some special attention.
In machines capable of a given small number of reduction modes, image exposure variations, in the prior art, were handled by arranging the exposure in a base mode to be relatively uniform, and then substituting a different mask, light stop or aperture, for each different mode to maintain the uniformity of exposure. However, as can be realized, when the number of reduction modes is increased to such a point that the reduction capability is essentially continuous the requirement to provide different masks, light stops or apertures, for each reduction mode, renders the system unmanageable in terms of equipment size, cost or maintainability. Accordingly, there has been a desire for achieving the capability of essentially continuously variable reduction, while maintaining image exposure relatively constant in a simple and inexpensive manner.
A system capable of achieving some of these goals is shown in Allis U.S. Pat. No. 4,057,342, issued on Nov. 8, 1977. This patent discloses a copying system with a pair of apertures (light stops, masks, slits, etc.) located in the optical path and capable of operating in a base mode and a reduction mode. The patentee recognized that additional reduction modes could be employed and, while image exposure variations would occur, the exposure system would provide a degree of correction. The patentee also indicates, however, that a slit appropriate for a base mode or nonreduction mode of operation would probably not be adequate for reduction mode of operation and correspondingly, a slit provided for uniform illumination in a reduction mode of operation would not provide proper operation in a base or nonreduction mode or in a different reduction mode.
It is an object of the present invention to provide a stationary aperture, mask or slit for exposure control which is applicable not only to a nonreduction mode of operation but also applicable to continuous reduction modes of operation. It is a further object to make the aperture symmetrical thus greatly relaxing the manufacturing and alignment tolerance requirements associated with prior art nonsymmetrical apertures.